360-degree video, also known as panoramic or immersive video, is a critical component in the Virtual Reality (VR) ecosystem. 360-degree videos, provide users with a panoramic view that allows the viewer to freely control their viewing directions during video playback. Spherical videos are recorded by omnidirectional cameras or camera array systems (e.g., FACEBOOK® Surround 360). The camera array simultaneously records all 360 degrees of a scene that can be “wrapped” onto a 3D sphere, with the camera array at its center. Spherical videos provide users with panoramic views and create a unique viewing experience in particular when used in combination with the 3D video technology. 360-degree videos are becoming increasingly popular on commercial video content platforms such as YouTube, Facebook, and Periscope. In a typical 360 video system, a user wearing a VR headset can freely change her viewing direction. Technically, the user is situated in the center of a virtual sphere, and the panoramic contents downloaded from video servers are projected onto the sphere (e.g., using equi-rectangular projection).
When watching a spherical video, a viewer at the spherical center can freely control her viewing direction, so each playback creates a unique experience. Normally, a player displays only a visible portion of a spherical video, known as a field of view (FoV). The user's viewport (visible area) is determined by her viewing direction (in latitude/longitude) and the FoV of the VR headset in real time. The FoV defines the extent of the observable area, which is usually a fixed parameter of a VR headset.